The present disclosure relates generally to imaging members, such as layered photoreceptor devices, and processes for making and using the same. The imaging members can be used in electrophotographic, electrostatographic, xerographic and like devices, including printers, copiers, scanners, facsimiles, and including digital, image-on-image, and like devices. More particularly, the embodiments pertain to an imaging member or a photoreceptor that incorporates specific materials, namely thiophosphates, into the anticurl back coating (ACBC) layer.
Electrophotographic imaging members, e.g., photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, charge is generated by the photoactive pigment, and under applied field charge moves through the photoreceptor and the charge is dissipated.
In electrophotography, also known as xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment move under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. In addition, the imaging member may be layered. These layers can be in any order, and sometimes can be combined in a single or mixed layer.
Typical multilayered photoreceptors have at least two layers, and may include a substrate, a conductive layer, an optional charge blocking layer, an optional adhesive layer, a photogenerating layer (sometimes referred to as a “charge generation layer,” “charge generating layer,” or “charge generator layer”), at least one charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generation layer (CGL) and the charge transport layer (CTL). Enhancement of charge transport across these layers provides better photoreceptor performance.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits, on the imaging members.
In multilayered imaging members, the CTL is usually the last layer to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 120° C., and finally cooling it down to room ambient temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing application of the CTL coating through drying and cooling processes, exhibition of spontaneous upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the CTL in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the CTL does therefore have a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature. The exhibition of imaging member curling after completion of CTL coating is due to the consequence of the heating/drying/cooling processing.
To offset the curling, an anticurl back coating is then applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness. Curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. An anticurl back coating having a counter curling effect equal to and in the opposite direction to the applied layers is applied to the reverse side of the active imaging member to eliminate the overall curl of the coated device by offsetting the curl effect which is arisen from the mismatch of the thermal contraction coefficient between the substrate and the CTL, resulting in greater CTL dimensional shrinkage than that of the substrate.
Although the anticurl back coating is needed to counteract and balance the curl so as to allow the imaging member web to lay flat, nonetheless, common formulations used for anticurl back coatings have often been found to provide unsatisfying dynamic imaging member belt performance under a normal machine functioning condition; for example, exhibition of excessive anticurl back coating wear and its propensity to cause electrostatic charge buildup are the frequently seen problems that prematurely cut short the service life of the photoreceptor belt and require its frequent costly replacement in the field.
Moreover, high surface contact friction of the anticurl back coating against all these machine subsystems is further been found to cause the development of electrostatic charge buildup problem. In many machines, the electrostatic charge builds up due to high contact friction between the anticurl back coating and the backer bars is seen to significantly increase the frictional force to the point that it requires higher torque from the driving motor to pull the belt for effective cycling motion. In full color electrophotographic machines, using a 10-pitch photoreceptor belt, this electrostatic charge build-up can be extremely high due to large number of backer bars used in the machine.
In an effort to resolve the problems associated with the anticurl back coating, one known wear resistance anticurl back coating formulated for use in the printing apparatuses includes organic particles reinforcement such as the utilization of polytetrafluoroethylene (PTFE) dispersion in the anticurl back coating polymer binder. PTFE particles are commonly incorporated to reduce the friction between the anticurl back coating of the belt and the backer bars. The benefit of using this formulation is, however, outweighed by the instability of the PTFE particle dispersion in the anticurl back coating solution. PTFE, being two times heavier than the coating solution, forms an unstable dispersion in a polymer coating solution, commonly a bisphenol A polycarbonate polymer solution, and tends to settle with particles flocculate themselves into big agglomerates in the mix tanks if not continuously stirred. The difficulty of achieving good PTFE dispersion in the coating solution poses a problem, because it can result in an anticurl back coating with insufficient and variable or inhomogeneous PTFE dispersion along the length of the coated web, and thus, inadequate reduction of friction over the backer bars in the copiers or printers. This causes significant complications in the larger copiers or printers, which often include so many backer bars that the high friction increases the torque needed to drive the belt. Consequently, two driving rollers are included and synchronized to prevent any registration error to occur. The additional components result in high costs for producing and using these larger printing apparatuses. Thus, if the friction could be reduced, the apparatus design in these larger printing apparatuses could be simplified with less components, resulting in significant cost savings.
Some anticurl back coating formulations are disclosed in U.S. Pat. Nos. 5,069,993, 5,021,309, 5,919,590, 4,654,284 and 6,528,226. However, while these formulations serve their intended purposes, further improvement on those formulations are desirable and needed. More particularly, there is a need, which is addressed herein, for a way to create an anticurl back coating formulation that has intrinsic properties to minimize or eliminate charge accumulation in photoreceptors without sacrificing the other electrical properties.
The term “electrostatographic” is generally used interchangeably with the term “electrophotographic.” In addition, the terms “charge blocking layer” and “blocking layer” are generally used interchangeably with the phrase “undercoat layer.”